Pressure-sensitive adhesives (PSAs) constitute one group of adhesives and are often used in the form of PSA tape, PSA label or the like obtained by coating PSA onto a substrate and curing. Such tapes and labels are typical of PSA articles familiar in everyday life. These articles are used as labels for identification, for packing of things, for binding a plurality of parts, and in many other applications.
Several base materials are known for PSAs and generally classified into rubber, acrylic, and silicone bases. Rubber base PSAs are general-purpose base materials which are known from the past, and frequently used as general-purpose tape and other products because of low price. Acrylic base PSAs are based on polyacrylates and have better chemical properties than the rubber base PSAs, and are applicable to higher performance PSA products. Silicone base PSAs are composed of high viscosity silicone gum and silicone resin wherein a multiplicity of siloxane bonds are contained in the backbone. Thus the silicone PSAs have a variety of excellent characteristics such as heat resistance, freeze resistance, weather resistance, chemical resistance and electric insulation.
By virtue of such excellent characteristics, the silicone PSAs are used as industrial high-performance tapes including heat resistant tape, masking tape, and flame retardant mica tape. They are often used in a rigorous environment since they exert their own characteristics even under severe service conditions. Recently the demand for silicone PSAs is expanding. This is accounted for by a rapid enlargement of the market of products having touch panels (or screens) mounted such as smartphones and tablet computers. Since touch panels are typically operated by directly touching with human fingers, a screen protective film is attached to the surface of a touch panel on use, for protecting the display from smear and flaw. The screen protective film includes a PSA layer which is mostly made of silicone PSA. This application is due to the characteristics of silicone PSAs including good wetting to adherends and ease of reworking.
The substrate for the screen protective film is typically plastic film. Among others, PET and other polyester films having transparency are used. However, the plastic films are said to be less adherent to PSAs than paper substrates. This is presumably because plastic films have a flat surface and weaken the anchoring effect of PSA biting into the substrate as compared with paper substrates having microscopic irregularities. Poor adhesion may raise problems, for example, back transfer upon winding in roll form, and that the PSA layer is left on the adherend when the PSA film is peeled from the adherend after the lapse of an ample time from the attachment of PSA to the adherend.
From the past, several measures were taken to improve adhesion. Typical measures are by using more adherent substrates, and treating substrates with corona discharge. Also primer treatment is on widespread use. Primers for silicone PSAs are developed as disclosed in Patent Documents 1 to 3. While the primer treatment is quite effective for improving adhesion, its serious drawback is the addition of an extra coating step which is deleterious to the cost and productivity. One effective measure is to add an adhesion-promoting component to silicone PSA, whereby adhesion is established by a single coating step. Although such additives are known from Patent Documents 4 to 6, there is still a need for further improvement in adhesion. Patent Document 7 describes an additive which provides selective adhesion to organic resins. This additive is added to silicone rubber compositions. Patent Document 7 refers nowhere to silicone PSAs.